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EVALUATION OF THE IMMUNE RESPONSE
– Analyse samples to: - Determine the titres of SP antibodies against homologous vaccine virus (no antibodies to FMDV should be found at day 0). It is also advisable to measure SP antibody titres against field strain(s) to measure protection against circulating virus. - Determine the presence of NSP (NSP antibodies should be absent throughout the field trial). – Calculate the proportion (and its confidence interval) of animals that developed specific antibody titres (or the proportion of animals developing an antibody titre equal to or above a cut-off considered to be protective). – Calculate the mean specific antibody titre at the different time points. The evaluation is successful only if animals are negative for both SP and NSP antibodies at the start. NSP seroreactivity at any time point indicates possible infection or lack of purity of the vaccine used. The results of such a field study should provide information (i) about the expected proportion of animals that will develop a specific level of antibodies following the administration of a single dose of the vaccine, (ii) to evaluate the effect of a booster dose, and (iii) about the duration (and level) of specific antibody titres over time. In combination with vaccine coverage data (if available), it can be used to estimate the expected proportion of animals with a specific level of antibodies at the population level. This partly addresses the aims of the studies indicated in section 3.5, although it is likely that in countries endemic for FMD the immune status of a population will be the combined effect of current or past vaccination programmes and previous exposure to field virus. In addition, wider field studies are needed to detect regional differences in vaccine application and induced immunity. The evaluation of the serological titres against a specific level of antibodies (considered to be protective) can also be interpreted as vaccine efficacy that in this case will correspond to the proportion of individuals that have developed an immune response equal to or above the protective titre. Few countries may be in a position to establish a serological titre cut-off point to discriminate between highly and less protected animals (as is done in South American countries). However, some countries may have useful information from the vaccine manufacturer or from experts on what level of antibodies may be considered acceptable. In such circumstances, the quantitative evaluation of the mean antibodies titre (and its 95% confidence interval) may help to establish provisional cut-off values. Foot and mouth disease vaccination and post-vaccination monitoring. Guidelines
3.5 Post-vaccination monitoring to assess immunity at population level The overall population immunity is the proportion (percentage) of animals with immunity in the whole population susceptible to FMD, or at least that part of it that has been targeted for FMD control. This is a function of the vaccine coverage and the proportion of animals that responded to vaccination, as well as reflecting other sources of immunity, namely infection, earlier vaccination or maternally derived antibodies. In countries embarking on FMD control where infection is still common, significant levels of post-infection immunity may be anticipated (commonly 15–30% or greater), whereas in countries at later stages of FMD eradication, post-infection immunity is unlikely to be a significant component of population immunity. It has been already mentioned when introducing vaccine coverage, how important it is to be clear, when designing and interpreting sero-surveys for population immunity, whether the aim is to sample only vaccinated animals or the entire population. In the example illustrated (Fig. 4), the whole population is a total of 30 cattle. The population eligible for vaccination is a subset of this population comprising 24 cattle. Of these 24 cattle, 20 are vaccinated and 14 have sufficient antibody against FMD (cattle surrounded by a green border). These antibodies may be due to either vaccination or infection and it is possible to distinguish between the two if both SP and NSP testing is performed, as vaccination should induce only SP antibodies, whereas infection will induce both SP and NSP antibodies. Some possible reasons for having non-immune cattle in the vaccinated population are: – not vaccinated, despite being eligible – not at home during campaign, too wild to vaccinate, late pregnancy, owner not cooperative; – not vaccinated because ineligible (e.g. below minimum age); – vaccinated but no immune response – depends on potency of vaccine, application of vaccination (low dose, spilled dose), shelf life of vaccine, cold chain. Reasons for animals not being part of the vaccinated population include: – insufficient vaccine doses available; – cattle not eligible for vaccination, for example too young;
